This research is focussed on the redifferentiation of the intestinal epithelium of mammals from the suckling to the mature state. Our current aim is to dissect the redifferentiation process into discrete steps that may be targets of specific hormones at specific times. Working primarily with rats, we are examining the effects of hypophysectomy, thyroidectomy, and adrenalectomy in early stages, with glucocorticoids, thyroxine, growth hormone, and other hormones being administered before and/or during the period (ca. 18-22 days) in which maturation of the intestinal epithelium culminates. The aspects of intestinal maturation that we observe and investigate include: growth and cell kinetics; accumulation of membrane-bound enzymes of the brush border, especially alkaline phosphatase, maltase, and sucrase; differentiation of cell ultrastructure, with particular attention to the Golgi membranes; uptake and distribution, as revealed by electron microscopy, of tritiated hexoses which may be precursors of the polysaccharide componets of the glycoprotein enzymes that become attached to the brush border. By combining biochemical data with ultrastructural indications of cellular activity at different stages of development, under a variety of conditions of endocrine deprivation and hormone replacement, we hope to be able to correlate evidences of intracellular activity with specific alterations of the brush border membrane, and to relate these in turn to the regulatory action of glucocorticoid or thyroxine.